What is the next step in management for a fetus with a single umbilical artery detected on ultrasound at 18 weeks gestation?

Management of Single Umbilical Artery at 18 Weeks Gestation

For an isolated single umbilical artery detected at 18 weeks gestation, close monitoring during pregnancy (Option D) is the correct answer, consisting of third-trimester ultrasound for growth assessment and consideration of weekly antenatal surveillance beginning at 36 weeks. 1, 2

Genetic Testing is NOT Required

• No additional evaluation for aneuploidy is recommended for isolated single umbilical artery, regardless of whether previous aneuploidy screening was negative or declined. 1, 2
• Isolated single umbilical artery shows no increased risk of aneuploidy, making genetic testing unnecessary. 2, 3
• Diagnostic testing for aneuploidy is not recommended solely for an isolated soft marker following negative screening. 2

Important caveat: If the single umbilical artery occurs with other structural abnormalities, aneuploidy risk increases dramatically to 4-50%, and genetic testing would then be indicated. 4, 3

Fetal Echocardiography is NOT Routinely Required

• If the required cardiac views on the anatomy scan are adequately visualized and normal, fetal echocardiography is not routinely warranted. 2, 4
• A comprehensive assessment of cardiac anatomy should be performed on the detailed anatomy ultrasound, as cardiovascular anomalies are strongly associated with single umbilical artery (odds ratios 5.9-20.3). 2, 3
• Particular attention should also be paid to the renal system, which shows a 3-fold increased risk of anomalies (approximately 16% of isolated cases). 3

Key distinction: The comprehensive cardiac assessment on the anatomy scan is adequate; a separate fetal echocardiography is only needed if cardiac views are inadequate or abnormal findings are suspected. 2, 4

Required Surveillance Protocol (Close Monitoring)

Third-Trimester Ultrasound

• Schedule a third-trimester ultrasound examination at 32-36 weeks to evaluate fetal growth. 1, 2, 4
• This is essential because neonates with isolated single umbilical artery have increased rates of growth restriction. 2
• Single umbilical artery is associated with a nearly 5-fold increased risk of stillbirth (OR 4.80,95% CI 2.67-8.62). 4, 3

Antenatal Surveillance

• Consider weekly antenatal fetal surveillance beginning at 36 0/7 weeks of gestation. 1, 2, 4
• This surveillance addresses increased risks of stillbirth, polyhydramnios, oligohydramnios, placental abruption, cord prolapse, and perinatal mortality. 2

If Growth Restriction Develops

• Perform serial umbilical artery Doppler assessment to evaluate for deterioration. 4
• Weekly cardiotocography testing is suggested after viability if growth restriction is diagnosed. 4
• Delivery timing would then be dictated by Doppler findings, ranging from 30-39 weeks depending on severity. 4

Postnatal Considerations

• Notify the pediatric provider at delivery of the prenatal findings. 2, 4
• Postnatal examination revealed structural anomalies in up to 7% of infants with prenatal diagnosis of isolated single umbilical artery. 2, 4

Common Pitfalls to Avoid

• Do not order genetic testing reflexively - isolated single umbilical artery carries no increased aneuploidy risk. 1, 2
• Do not order fetal echocardiography routinely - only if cardiac views on anatomy scan are inadequate or abnormal. 2, 4
• Do not provide routine antenatal care without enhanced surveillance - third-trimester growth assessment and consideration of weekly testing at 36 weeks are essential. 1, 2
• Do not assume "isolated" without thorough anatomic survey - carefully evaluate cardiac and renal systems, as 7% may have occult anomalies detected postnatally. 2, 5